Microporous membranes include thin sheets and hollow fibers generally formed from synthetic thermoplastic materials and having a substantially continuous matrix structure containing pores or channels of small size. The size range for pores of "microporous membrane" is not precisely defined in the art, but is generally understood to extend from about 0.05 to about 10 micrometers.
A wide variety of polymeric materials have been employed to produce microporous membranes. Examples of these polymers include: polyolefins such as low density polyethylene, high density polyethylene, and polypropylene; vinyl polymers; acrylic polymers such as polymethylmethacrylate and polymethylacrylate; oxidation polymers such as polyphenylene oxide; fluoropolymers, such as polytetrafluoroethylene and polyvinylidene fluoride; and condensation polymers such as polyethylene terephthalate, nylons, polycarbonates and polysulfones.
Despite the wide variety of polymers employed in the production of microporous membranes, there have been no reports in the literature of the production of microporous UHMW-PE membranes employing conventional extrusion equipment and/or methodology. This is particularly noteworthy because UHMW-PE is known to have outstanding properties. Like other polyethylenes, it possesses a nonadherant surface, low coefficient of friction and good chemical resistance. In addition, UHMW-PE exhibits very high abrasion resistance and impact resistance. UHMW-PE also has outstanding resistance to stress cracking.
Because of its outstanding properties, there has been much effort directed to forming articles from UHMW-PE, but processing UHMW-PE has proven to be very difficult. This polymer does not flow under ordinary conditions; therefore, it has been formed into shaped articles under high pressure, high temperature and very long heating times. Such conditions usually degrade the properties of UHMW-PE. Extrusion on typical extrusion equipment has not usually been possible and processing has often been based on powder metallurgy techniques, such as sintering.
A common technique which has been employed for processing molten UHMW-PE polymer is compression molding. Ram extrusion has also been employed. Twin screw extrusion with corotating screws has also been tried with a special extrusion grade material but has been increasingly abandoned because of problems with voids and internal stresses in the molded parts. See Birnkraut, W. H., Braun, G. and Falby, J., "Ultrahigh Molecular Weight Polyethylene - Processing and Properties", J. Appl. Poly. Sci.: Appl. Poly. Sym., 36,79-88 (1981).
Compression molding of UHMW-PE into films has been reported by Smith and Lemstra. See, Smith, P. and Lemstra, P. J., "Ultra-Drawing of High Molecular Weight Polyethylene Cast From Solution", Colloid Polym. Sci., 258, 891 (1980). In this work, compacted UHMW-PE powder was compression molded at 160.degree. C. into films having a thickness of 0.16 mm. These films were solidified by quenching them to room temperature.
In addition, Smith and Lemstra have reported production of UHMW-PE films by casting of the polymer from relatively dilute solutions in decalin. See Smith and Lemstra, ibid. In this casting technique, polymer solution was prepared at 160.degree. C. under nitrogen and was stabilized by 0.5% w/w of the antioxidant di-t-butyl-p-cresol. Upon quenching of the cast dilute solution, a polymer gel was generated. Solvent was allowed to evaporate at room temperature leaving an UHMW-PE film with a thickness of 0.14 mm containing 4% w/w of decalin. The last trace of the solvent was removed by extraction with ethanol.
The films reported by Smith and Lemstra apparently are porous although little is reported about the morphology of the porous structure. In addition, there are no reports of whether these films were permeable. The films produced were subjected to ultra-drawing in order to improve their mechanical properties. See Smith and Lemstra, ibid; Smith, P., Lemstra, P. J. and Booij, H. C., "Ultra-Drawing of High-Molecular Weight Polyethylene Cast From Solution. II. Influence of Initial Polymer Concentration", J. Polym. Sci., Polym. Phys. Ed., 19, 877 (1981); and Matsuo, M. and Manley, R. S. J., "Ultra-Drawing at Room Temperature of High Molecular Weight Polyethylene", Macromol., 15, 985 (1982).
A patented technique for forming porous structures, including sheets, from UHMW-PE is disclosed in U.S. Pat. No. 3,954,927 issued to Duling et al. In the Duling et al. patent, a method of forming a porous UHMW-PE structure is disclosed which comprises establishing a uniform heterogeneous composite of UHMW-PE particles and a hydrocarbon; forming the heterogeneous composite into an object of the desired shape; and subsequently heating the object to a temperature above the melting point of the polymer until the polymer particles have completely fused. The formed structure is cooled and the hydrocarbon is removed leaving a porous object. Although the Duling patent describes the production of porous articles, the technique is unconventional and limited in application.
Thus, despite the potential advantages offered by UHMW-PE as a material of fabrication for microporous membranes, particularly permeable microporous membranes, such advantages have not yet been realized. Permeable microporous membranes formed from UHMW-PE have not been reported. Similarly, the use of conventional extrusion technology and/or equipment in the formation of UHMW-PE has not been reported.